This invention relates to a method of and apparatus for controlling the operation of a variable speed gearing capable of preventing fluctuations in the power on a transmission line, when a fault, such as a one-line ground or a three-line ground, has occurred in the transmission line, and thereby improving the reliability of power generation facilities.
FIG. 8 schematically shows an example of an operation controller for a variable speed gearing, such as an induction generator-motor.
In FIG. 8, the primary circuit of an induction generator-motor 5 is connected to a transmission line 1 and a transmission line 2 via breakers CB11, CB12 and breakers CB21, CB22, a breaker CB3, a main transformer 3, disconnectors DS1, DS2, and a parallel breaker 4 in that order.
An exciting current (secondary exciting current) to the secondary circuit 10 of the induction generator-motor 5 is produced in such a manner that the alternating current on the transmission lines 1 and 2 inputted via an exciting power supply breaker 6 and an exciting transformer 7 is converted into a direct current by a power rectifier 8 in the presence of a power rectifier control signal 13 from a variable speed controller 11 and then the direct current is converted into an alternating current of variable frequency by a power inverter 9 in the presence of a power inverter control signal 14 from the variable speed controller 11.
The application of the secondary exciting current enables the induction generator-motor 5 to be operated at variable speed in synchronization with the alternating current on the transmission lines 1 and 2.
On the other hand, when a one-line ground or a three-line ground has occurred at a fault point 19 on the transmission line 2, the induction generator-motor 5 supplies a fault current to the transmission-line fault point 19. As a result, the fault current flows in the primary circuit of the induction generator-motor 5. The fault current then produces electromotive force in the secondary winding of the induction generator-motor 5, resulting in excess current flowing in the secondary circuit of the induction generator-motor 5.
FIG. 9 is a waveform diagram to help explain an example of excess current in the secondary circuit 10 of the induction generator-motor 5 when an fault has occurred in the transmission line.
In FIG. 9, the current waveform 21 of the secondary circuit 10, which is generally of a three-phase balanced current, varies with time in the direction of the arrow shown by t.
On the other hand, when a fault has occurred in the transmission line 2, the current in the secondary circuit 10 of the induction generator-motor 5 increases, because the balance of the three-phase current is lost at the transmission line fault point 22 due to the induced voltage in the secondary circuit 10 caused by the fault current supplied from the induction generator-motor 5.
The current in the secondary circuit 10 of the induction generator-motor 5 is sensed by a secondary current sensor 15 provided for each of the three phases of the secondary circuit 10. Excess current is sensed, provided that an excess current sensor 16 senses that the current has exceeded a preset value. The excess current sensor 16 starts to operate at an excess current sensing point 23.
Then, when the excess current sensor 16 has started to operate, the variable speed controller 11 judges that it is impossible to continue the operation of the induction generator-motor 5 and outputs a generator trip signal 20 to a generator controller 12, thereby tripping the parallel breaker 4 and exciting power supply breaker 6, which not only disconnects the induction generator-motor 5 from the transmission lines 1 and 2 but also stops the operation of the induction generator-motor 5.
In FIG. 9, the disconnected state of the induction generator-motor 5 is shown at the generator trip point 24.
On the other hand, when a transmission line fault has occurred, a transmission protective device 17 and a transmission protective device 18 provided in the transmission line 2 operate, closing the breakers CB21, CB22 in the transmission line 2 again, restoring the transmission line 2 to normal conditions.
At this time, when the ground fault at the transmission line fault point 19 is transient and lasts for a short time, the re-closing of the circuit is successful, restoring the transmission line 2 to the state before the fault occurred. When the ground fault at the transmission line fault point 19 has lasted for a long time, the re-closed circuit is broken finally, tripping the breakers CB21, CB22 at both ends of the transmission line fault point 19 to disconnect the fault point 19 from the transmission line 2, which keeps the transmission line 2 in good condition.
With such a method, when excess current in the secondary circuit 10 of the induction generator-motor 5 has been sensed, it cannot be judged whether the fault has occurred in the transmission line 2 or in the secondary circuit 10 of the induction generator-motor 5. For this reason, the operation of the induction generator-motor 5 is stopped in the presence of excess current in the secondary circuit 10, although the fault in the transmission line 2 might be remedied.
As described above, with a conventional variable speed gearing operation controller, when a fault, such as a one-line ground or a three-line ground, has occurred at the fault point 19 in the transmission line 2, the transmission line protective devices 17 and 18 provided on the transmission line 2 side operate, restoring the transmission line 2 to normal conditions by the re-closing of the circuit or by the final cutoff of the re-closed circuit, which stops the operation of the induction generator-motor 5 in the presence of excess current in the secondary circuit 10, although the fault in the transmission line might be remedied.
Namely, with such a method, although the induction generator-motor 5 itself has not gone wrong, the operation of the induction generator-motor 5 is stopped. As a result, the induction generator-motor 5, which may be operated without being disconnected from the transmission line, is disconnected, preventing the supply of power. Furthermore, the power cannot be supplied to the power system which is short of power due to the fault in the transmission line 2, causing fluctuations in the power on the transmission line 2.